Therefore also the good unmixed with evil is the greater good. But God makes always what is best, much more than nature does. Therefore in things made by God there is no evil.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil God
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17611/pg17611-images.html

Context

“If therefore evil is a being in things, it follows that evil is a thing, which is contrary to what has been said (A. 1) .
Obj. 3: Further, "the white unmixed with black is the most white," as the Philosopher says (Topic. iii, 4) . Therefore also the good unmixed with evil is the greater good. But God makes always what is best, much more than nature does. Therefore in things made by God there is no evil. On the contrary, On the above assumptions, all prohibitions and penalties would cease, for they exist only for evils.
I answer that, As was said above (Q. 47, AA. 1, 2) , the perfection of the universe requires that there should be inequality in things, so that every grade of goodness may be realized.” source